Minor Characters are characters who play a very small role in the show, characters with no names, and characters who are mentioned but not seen.
K.O. and Dendy's Classmates are the classmates of K.O. at Original County Public School. Three of them are officially named as Nanini, Genesis, and Bobo. They've made a flashback appearance in "I Am Dendy" with their debut appearance of "Mystery Science Fair 201X" and "The So-Bad-ical".
Old Man Jenkins
Old Man Jenkins, as his name is revealed in the credits of "Hope This Flies" and in a pow card, wears a yellow top with a yellow hat and his right arm is mechanical. His beard is the same color of his hair, white. He is voiced by Ian Jones-Quartey. His debut appearance was in "One Last Score" as his far distance render, which has a different appearance than his ordinary appearance (beginning from "Plazalympics" as his color palette consists of more of brown hue. He has a pointed, protruding nose as opposed of triangular shape, and he has no hair and no beard. According to his pow card, he is a level -1 and is afraid of taking bath.
Ms. Mummy's Mother: Dr. Sphinxon
Dr.Sphinxon looks like a human with a lion's tail, which is very different than her daughter. However, they both have an Ancient Egyptian motif. Her first appearance is in "Parents Day", and her second appearance is in "Radical Rescue" where it is revealed she runs a cat shelter. She is voiced by Constance Schulman.
Drupe's mother looks like an orange with a business woman-like outfit and has a purse that resembles a slice of watermelon. It is not known why she and her daughter are not the same kinds of fruit, as Drupe is a Strawberry. Her only appearance is in "Parents Day".
Brandon's parents bear resemblance to the Hanna-Barbera characters Yogi Bear and Cindy Bear. Their only appearance is in "Parents Day".
A small rain cloud with a briefcase. He rained on K.O. His only appearance was in "You're Level 100!".
Kappas were ineligible to create their own pow cards until the end of the episode "No More Pow Cards", where there were many kappas demonstrating heroic qualities. One of the unknown Kappas is level 0.1, the same level K.O. had up until "You're Level 100!".
Animals in Human-Suits
They are all animals that wish to be humans and wear human-like clothing and disguises to fit in. Their only appearance was in "We've Got Fleas".
Blue Guy in a Tie
He wears a small yellow tie while the rest of his body is blue. His only appearance was in "Plaza Prom".
Small Purple Cat
He is a small purple cat who appears in the crowd at the plaza prom in "Plaza Prom".
The man with the "Rad Dad" shirt is voiced by Dave Herman and is credited as "Rad Dad," the lady with the "le Blorp" shirt is voiced by Melissa Villasenor and is credited as "Rad Fan," and the zombie-like fans are voiced by Ben Jones and is credited as "Zombie Like Fan." They only appeared in "Everybody Likes Rad?".
Puck Reverie's Mother
The mother of Puck Reverie and the sister of Bernard only appeared in "Parents Day" albeit in a family photo. She is described as a lady wolf-man, and was previously married to an angel. Her fur is purple, much like her brother's.
Unnamed Wizard Villain
He fought against Silver Spark once. He only appeared in "Know Your Mom".
Prostratum is a former member of the Kaktus Krew who is mentioned by K.O. in the episode, "Know Your Mom". The way the scene plays out when K.O. tries to call Prostratum, it seems as if they passed away.
A Giant borg is a minor character in OK K.O.! Let's Be Heroes. This character first appeared in "Red Action to the Future". It is silver and diamond colored, and is big like Chameleon Sr. Its abilities and powers are laser beams and force field. He can shoot lasers out of his eyes and he can protect himself from his opponent's attacks.
Stickman was a guest hero character in "Crossover Nexus", originally appearing in storyboards of the 22-minute version of the episode but was scrapped when the episode was cut down to 11 minutes. Stickman has the normal build of a normal stickman, but with two muscular arms, and a small yellow cape. He has no face and his body is colored red. Due to only appearing in storyboards in "Crossover Nexus", his personality is currently unknown. However, he is seen to be friendly and sportsman-like, calling K.O, Garnet, Ben, and Raven a 'pretty creative team' with a heroic voice, as shown by his dialog box.